Nickel-iron alloys and nickel-cobalt-iron alloys of controlled composition have long been known and used in applications in which controlled, low expansion characteristics are desired. The Eiselstein et al. U.S. Pat. No. 3,157,495 introduced to the art age-hardenable, controlled expansion alloys having high strength at room temperature and at elevated temperatures. The availability of such alloys caught the attention of gas turbine engine builders, particularly those building aircraft engines. Due to the requirements for strength, ability to resist loads for long times at elevated temperature, notch resistance, etc. imposed by the engine builders in respect of parts to be used in engines, extensive testing was conducted upon the alloys provided in accordance with U.S. Pat. No. 3,157,495 and certain deficiencies in properties were noted. A divergence of views has arisen as to how such deficiencies should be remedied. A succession of patents directed to modifications of the alloys has resulted, of which U.S. Pat. Nos. 3,705,827, 4,006,011, 4,026,699, 4,066,447 and 4,200,459 can be mentioned. U.S. Pat. No. 3,971,677 and U.K. Pat. No. 1,411,693, which are directed to cast products, can also be mentioned. Testing programs have revealed that the failure mechanism encountered in notched specimens in these essentially chromium-free alloys is that of stress-corrosion due to oxidation or oxygen embrittlement. Thus, alloys which have poor notch strength in air have excellent notch strength when tested in vacuum. It has also been observed that, due to relaxation effects, stress-rupture ductility and notch resistance in some alloys may be satisfactory at temperatures on the order of 1200.degree. F. or 1300.degree. F., but inadequate at 1000.degree. F.
Previous high aluminum, controlled expansion alloys had significant shortcomings of notch-rupture strength, especially when testing recrystallized grain structures or when thermomechanically processed structures were tested transverse to the direction of work. Such alloys showed 100 hr. notch strength of only about 50 Ksi (1 Ksi=1,000 pounds per square inch) or less at 1000.degree. F.
It is desirable to improve such 100 hr. notch-rupture strength of controlled expansion alloys to at least 100 ksi. Further, it is sometimes advantageous for controlled expansion alloys to exhibit notch ductile behavior; i.e., where notch bar rupture life exceeds smooth bar rupture life.
It is known that the aging treatments designed to produce required properties in these age-hardenable alloys will vary depending upon the properties to be emphasized. Thus, heat treatments designed to maximize elevated temperature strength and notch strength are generally longer and at higher temperatures than those designed to maximize short-term strength and the former are, in fact, overaging treatments. For example, in the low-aluminum alloys described in U.S. Pat. No. 4,200,459 it is now known that such overaging treatments are necessary to develop good notch rupture strength at 1000.degree. F. It has become desirable to increase the short-term tensile properties of low Al controlled COE alloys while retaining the good rupture strength previously attained only by overaging.
Economic pressure has introduced a need to shorten overall heat treating times. In addition, there is a need to provide alloys which exhibit good notch strength after exposure to high solution treating temperature which may, for some purposes such as brazing, be 1900.degree. F. or higher.
It is to the solution of these and other problems that the present invention is directed.